Commercially available DC motors for example used in fans can be classified into three types of constant revolving speed, adjustable revolving speed with external voltage, and adjustable revolving speed with temperature. Generally speaking, different control means are adapted to different types of motors. If digital control means can be incorporated into the design of a DC motor to control its revolving speed, the precision of the speed control can be enhanced. On the other hand, it will be more preferred if a cost-effective digital control device is available for precisely controlling the revolving speed of a DC motor.